Recent terrorist attacks in the United States have increased the awareness of the need for ways to protect drinking water supplies. Source waters for civilian populations and military facilities are vulnerable to such attacks. There is therefore a need for improved water quality sensor systems that accurately detect toxic materials in real-time in a water source and transmit an indicative signal. Currently available systems for continuous, automatic monitoring of water quality by sensing changes in photosynthetic activity have no provision for availing dark adaptation of photosynthetic organisms before measurements are taken.